The surface typing-antigens of nontuberculosis mycobacteria are lipids containing peptide and carbohydrate. Those in the Mycobacterium avium-M. intracellulare-M. scrofulaceum (MAIS) serocomplex are C-mycosidic peptidoglycolipids (PGLs), and the PGLs elaborated by different serovars have an individually characteristic oligosaccharide linked to a constant monoglycosyllipopeptide. These oligosaccharides can be released from the PGLs by alkaline reductive cleavage in a form suitable for chemical analysis. It is proposed to carry out detailed investigation on the oligosaccharide units from many serovars in order to fully comprehend the molecular basis of sero-specificity and to furnish the necessary information for the preparation of semi-synthetic antigens. Parallel studies will be conducted on other species of nontuberculosis mycobacteria. The possibility that the multiplicity of MAIS serovars in Nature is due to lysogenic conversion of PGL antigens will be examined, in conjunction with the mechanism of their biosynthesis. Tentative evidence that the PGLs occur as a filamentous microcapsule responsible for certain pathogenic and physiological features of nontuberculosis mycobacteria will also be examined.